Conventional piezoelectric speakers are known as small-sized and low-current drive acoustic equipment that uses a piezoelectric body for an electro-acoustic conversion element, and they are used as acoustic generation devices that are installed in small-sized electronic devices, for example, mobile computing devices, or the like.
Generally, acoustic generators that use a piezoelectric body for an electro-acoustic conversion element have a structure such that an electrode is formed on a piezoelectric body by using a thin silver film, or the like, and a piezoelectric element is attached to a metallic vibration plate. In this kind of acoustic generators, an alternate-current voltage is applied to the piezoelectric element so that the form distortion occurs in the piezoelectric element, and the form distortion of the piezoelectric element is transmitted to the metallic vibration plate for vibration, whereby sounds are generated.
However, in the acoustic generators that have a structure such that the piezoelectric element is attached to the metallic vibration plate, the piezoelectric element that expands for vibration is constrained by the metallic plate whose surface area does not change so that surface-area flexion vibration occurs; therefore, acoustic conversion efficiency is low, and it is difficult to obtain the sound pressure characteristics in which the resonance frequency is low while it has a compact size.
For these problems, the applicant has proposed an acoustic generator that uses a resin film as a vibration plate instead of a metallic vibration plate (for example, see Patent Literature 1).
In this acoustic generator, a bimorph lamination-type piezoelectric element is sandwiched between a pair of resin films in its thickness direction, and the resin films are secured to a frame member in a tensioned state. Thus, acoustic conversion efficiency can be improved, and a high sound pressure can be generated.